This invention relates to improvements in containers used in the microwave heating of foods and, particularly, containers of the type having a conductive shield laminated or otherwise applied to the surface of the container to control the exposure of the packaged food to microwave energy and thus control the heating process. More particularly, the present invention relates to such a package in which the shield is formed from a combustible material such as paper, or a thermoplastic material which might melt under the influence of high temperature.
Typically, the food product is packaged in a dielectric tray which, in turn, may be inserted into an outer package which has a conductive shielding imprinted on or otherwise applied to the package. The conductive shielding is of a configuration to define radiation transparent apertures which are of a size and configuration dependent on the nature, size and shape of the food product in the tray. The outer package is provided on one surface (usually its outer surface) with the conductive shielding film. This can present some difficulties and danger, particularly when the outer package is formed primarily of a combustible material such as thin cardboard, paper or the like. The danger of fire results from the fact that slight imperfections in the shielding film may, sometimes, cause arcing. Although the arcing usually is of a short duration, the temperatures generated in the region of the arc far exceed the flash point of the combustible container. That may cause the cardboard or paper to ignite. Arcing is not uncommon and can result in a number of instances, for example, by a scratch mark or even a small pinpoint in the shielding film. Other irregularities in the shape or edges of the shielding material can have the same effect because such irregularities tend to concentrate the strength of the microwave field in those regions.
In accordance with the present invention, the sheet material from which the outer package is formed is coated or laminated on both surfaces of the sheet so that no significant portions of the combustible sheet will be exposed to the oxygen in the air. Thus, even if some arcing may occur, there is insufficient exposure of the sheet to the oxygen and combustion will not be supported. In the preferred embodiment of the invention, both sides of the dielectric sheet are coated with a metallic film which serves both to shield the food as well as dissipate rapidly heat generated by such arcing.
It is among the general objects of the invention to provide an improved shielding container for use in microwave heating.
Another object of the invention is to provide a microwave shielding container which has improved safety features and which will minimize any tendency for the container to burn as a result of arcing.